Downloading PHP content from another domain (safe way)?

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So, if this question has been asked before, I'm sorry. I'm not exactly sure what to search for.Introduction: All the domains I maintain now are hosted on my server, so I have not ran into this problem yet.I have created a structure, similar to WordPress, for uploading and editing images.I regularly create changes in the functions and upload them to a single folder. When the user logs in, the contents are automatically downloaded into their folder.What I am wanting to do:Now, say I have a user that is not hosted on my server. I cannot use copy(), but is there a safe and secure way to echo the contents of each php file (obviously, I can echo) into another file on the users server?For example:Currently I can copy from jasonleodurbin.com to geodun.com (same server), but say I want to copy jasonleodurbin.com/test.php to somedomain.com/test.php.I had some thoughts like give each user a private key and send that to a file like echo.php. echo.php will grab the contents of every file (that has been modified recently) and echo that to the screen. The requesting server would take that content and copy that into it's respective .php file.I assume I could send the key through GET, but since I have never dabbled into the security implications of anything (I am a hobbyist), I don't know how secure this is.Are there any suggestions or directions that someone could send me?I appreciate the help!